Byte-wise SWAP

#define SWAP(a, b, size)                                    \
    do {                                                    \
            register size_t _size = (size);                 \
            register char *_a = (a);                        \
            register char *_b = (b);                        \
                                                            \
            do {                                            \
                    char tmp = *_a;                         \
                    *_a++ = *_b;                            \
                    *_b++ = _tmp;                           \
            } while(--_size > 0);                           \
    } while(0)